thermochemistry Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on ...

, the Thomsen–Berthelot principle is a hypothesis in the

history of chemistry The history of chemistry represents a time span from ancient history to the present. By 1000 BC, civilizations used technologies that would eventually form the basis of the various branches of chemistry. Examples include the discovery of fire, ex ...

which argued that all

chemical changes A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products ...

are accompanied by the production of

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

and that processes which occur will be ones in which the most heat is produced. This principle was formulated in slightly different versions by the Danish chemist Julius Thomsen in 1854 and by the French chemist Marcellin Berthelot in 1864. This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades. This principle came to be associated with what was called the ''thermal theory of affinity'', which postulated that the heat evolved in a

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

was the true measure of its

affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Pa ...

Limitations

The experimental objections to the Thomsen–Berthelot principle include incomplete dissociation, reversibility, and spontaneous endothermic processes. Such cases were dismissed by orthodox thermochemist as outliers not covered by the principle, or the experiments were manipulated to fit it through with somewhat contrived justifications was later disproved. In 1873, Thomsen acknowledged that his theory might not have universal or definitive credibility. Later, under newly created

chemical thermodynamics Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. Chemical thermodynamics involves not only laboratory measure ...

framework, the principle was explained to only be valid as an idealization under extreme conditions (i.e.,

absolute zero Absolute zero is the lowest possible temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The absolute zero is defined as 0 K on the Kelvin scale, equivalent to −273.15 ° ...

). Thomsen openly admitted that his initial understanding was merely a close estimate of the reality, emphasizing that while chemical reactions typically release heat, this heat isn't always a trustworthy indicator of the strength of the bonds formed. On the other hand, Berthelot, was more resistant and continued to assert the validity of the principle until 1894.M. Berthelot, 'Le principe du travail maximum et l'entropie,' Comples rendus, 1894, 118, 1378-1392. In 1882 the German scientist

Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (; ; 31 August 1821 – 8 September 1894; "von" since 1883) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The ...

proved that affinity was not given by the heat evolved in a chemical reaction but rather by the maximum work, or free energy, produced when the reaction was carried out reversibly.

Limitations

References

See also